Optical fibers are often grouped into optical fiber ribbons in which the constituent fibers are held in a predetermined spatial relationship in order to facilitate connections. The required precision of positioning normally demands that the fibers are in close proximity to each other, for example, in contact with each other or structural glass fibers that may be used as spacers. The fibers are typically arranged in a common plane with an extruded common matrix material thereover, which arrangement can limit the cross-sections of extruded plastic material because of the stresses and distortion that would arise during cooling.
Optical fiber ribbons can formed with or without sub-units. The curing of a UV radiation-curable composition suitable for use as a sub-unit or common matrix material is essentially a polymerization of the UV curable material, whereby the material undergoes a transition from a liquid to a solid. Prior to application to an optical fiber or a sub-unit, the UV curable material comprises a mixture of formulations of liquid monomers, oligomer "backbones" with, e.g., acrylate functional groups, photoinitiators, and other additives. Photoinitiators function by: absorbing energy radiated by the UV or visible light source; fragmenting into reactive species; and then initiating a polymerization or hardening reaction of the monomers and oligomers. The result is, in general, a solid network of crosslinking between the monomers and oligomers that may include fugitive components after cure. The photoinitiator therefore begins a chemical reaction that promotes the solidification of the liquid matrix to form a generally solid film having modulus characteristics.
A key to the curing process is the reaction of a photoinitiator in response to UV radiation. A photoinitiator has an inherent absorption spectrum that is conveniently measured in terms of absorbance as a function of the wavelength of the radiated light. Each photoinitiator has a characteristic photoactive region, i.e., a photoactive wavelength range (typically measured in nanometers (nm)). Commercially available photoinitiators may have a photoactive region in the vacuum ultra-violet (VUV)(160-220 nm), ultra-violet (UV)(220-400 nm), or visible light (V-light)(400-700 nm) wavelength range. When the material is irradiated by a VUV, UV or V-light lamp, that emits light in the photoactive region, the material will cure.
Optical fiber ribbons are typically flat bodies that are rather fragile and have very different bending characteristics in relation to given bending directions. In particular, optical fiber ribbons can be susceptible to damage if pressure is applied to a part of the ribbon that is so twisted that the direction of its planar thickness is out of alignment in relation to the direction of the pressure.
An example of an optical fiber ribbon is shown in JP 09-197,205-A to Noriyuki et al, that discloses an optical fiber pair enclosed in a circular jacket with a substantially increased diameter. The declared purpose is to enable the formation of cables with a high fiber density by laying the jacketed ribbons about a strength member. Another example is U.S. Pat. No. 4,070,093 to Schwartz which shows plural optical fiber ribbons enclosed in a circular-section duct forming a part of a cable structure. U.S. Pat. No. 5,636,308 to Personne et al discloses optical cable structures in which individual fibers (not forming parts of a ribbon) are enclosed in cavities of a dumbbell shape in a circular body having other cavities for strength members. A viscous fluid occupies void spaces in the cavities to provide waterproofing and ensure free movement, with the incidental effect of making it impossible for the fibers to be precisely located so that they will have to be connected separately.